The pulse width of a laser typically increases with increased repetition rate (i.e., the rate at which pulses are emitted by the laser). This is because at high repetition rates the time to store energy in the laser rod prior to each pulse is short and at low repetition rates the time to store energy in the laser rod prior to each pulse is long. Hence, on a per pulse basis, there is great variation in the energy output and temporal pulse width as the repetition rate is varied.
This effect is due to the fact that the energy that can be extracted from a laser rod depends on the energy stored in the rod. For example, at a repetition rate of 30 kilohertz there are only about 33 microseconds available to store and open a Q-switch to allow a laser pulse to be emitted, whereas at 1 kilohertz there are about a thousand microseconds available to store and Q-switch. The gain in a laser is proportional to quantity of energy stored in the rod. Therefore, when a laser pulse is instigated at a low repetition rate it sweeps up much more quickly than it would at a higher frequency because there is more energy stored in the rod, resulting in a shorter temporal pulse width.
For a given energy per pulse the peak power varies inversely with the laser pulse width. Therefore, the peak power of a 300-nanosecond pulse is much less than the peak power of a 100-nanosecond pulse having the same total energy. The total energy per pulse delivered to the workpiece is typically controlled by a device that attenuates the beam; laser pulses at 1 kilohertz would be attenuated more than laser pulses at 10 kilohertz in order for the pulses in each instance to have the same total energy.
It is possible to widen laser pulses provided by a given laser at low repetition rates by lowering the energy stored in the laser rod when the laser is operated at low repetition rates. This can be accomplished by lowering the amount of energy that enters the rod from the laser pump. The Light Wave Electronics Model 110 laser works according to this principle.
It is also possible to ensure similar pulse widths at differing repetition rates by pumping energy into the laser rod prior to each laser pulse for about the same storage time period regardless of the repetition rate. After this high energy storage time but prior to opening of the Q-switch, the energy that is pumped into the laser rod is reduced to a level that is just above a threshold required to compensate for losses in the energy stored in the laser rod. This reduced energy level can be maintained until the Q-switch is opened to allow a pulse to be released from the laser rod.
General Scanning""s M320 pulsed laser system is an example of a system that does not ensure similar pulse widths at differing repetition rates. In this system, an acousto-optic modulator (AOM), is placed between the laser and the workpiece. As the laser scans over a workpiece, the acousto-optic modulator blocks laser pulses from impinging on the workpiece except when a laser pulse is needed to remove a link on the workpiece. In order to remove a link, the acousto-optic modulator allows a single pulse, emitted immediately after opening of the Q-switch, to impinge on the link. The acousto-optic modulator can allow only a fraction of the energy of the pulse to impinge on the link, as desired.
Togari et al., U.S. Pat. No. 5,719,372 describes a laser marking system in which laser pulses create holes in a workpiece that form a marking. Each emission period, during which the Q-switch is off (open), is sufficiently long to allow the laser to emit a primary emission pulse and a plurality of secondary emission pulses, all of which impinge upon the workpiece. The intensities of these primary and secondary emission pulses are less than the intensity of the single emission pulse that would be emitted if the emission period were shorter and the repetition rate kept the same. The low-power secondary emissions deliver extra energy to the workpiece. The patent claims that the low-power secondary emissions result in improved visibility of marking a lines in a workpiece that includes a resin film containing carbon.
One aspect of the invention features a pulsed laser system that includes a laser pump (e.g., a continuous wave (CW) pump), a laser rod, a reflector interposed between the laser pump and the laser rod, through which energy from the laser pump enters the laser rod, an output reflector through which energy is emitted from the laser rod, and a switch (e.g., a Q-switch) interposed between the laser rod and the output reflector. Further there is a control device, which may be external to the laser resonator. The Q-switch, when closed, causes energy to be stored in the laser rod and, when opened, allows energy to be emitted from the laser rod during an emission period. The control device allows a primary laser pulse emitted from the laser rod during the emission period to impinge on a workpiece and prevents at least a portion of secondary laser emission occurring after the primary pulse during the emission period from impinging on the workpiece.
The diode pumped laser technology according to the invention provides flexibility in and control over the pulse width, along with the repetition rate, in order to optimize performance. The invention makes it possible to use a laser that has short pulse widths at high repetition rates to process a workpiece (for example, to perform resistor trimming) at low repetition rates without emitting unduly short pulses. The low repetition rates may be especially useful for certain applications such as trimming high valued resistors.
The invention does not require any reduction in the output of the laser pump in order to provide wide pulses at low repetition rates. Thus, it is not necessary to redesign or otherwise accommodate the power supply electronics and feedback circuitry that are designed to ensure a stable output of the laser pump. Also, the invention does not require energy to be pumped into the laser rod at a reduced level during the portion of the emission period following emission of the primary laser pulse. Thus, the invention need not concern itself with errors that might be introduced into the total energy stored in the laser rod following emission of the primary laser pulse, which errors would be especially significant at low attenuation.
Because a control device is provided that prevents unwanted output emitted during the emission period after emission of the primary pulse from impinging on the workpiece, this portion of the laser output does not affect the temperature of the workpiece, and therefore does not affect measurements that might take place prior to each primary pulse, which may be temperature-sensitive, and does not affect performance of the workpiece. For example, in trimming of thick-film resistors, resistance measurements might take place immediately prior to each primary pulse. In micromachining of a semiconductor circuit on a silicon substrate, elimination of secondary pulses and a continuous wave output can prevent undue heating of the silicon substrate and thereby protect the silicon substrate against damage.
Another aspect of the invention features a method in which the pulsed laser system is operated over a range of repetition rates, so as to cause laser energy to be emitted during a plurality of emission periods at each repetition rate. At least a portion of the laser energy emitted during the emission periods is directed toward the target structure. The switch is closed for a fixed, predetermined period of time prior to each emission period regardless of repetition rate of the primary laser pulse within the range of repetition rates. The pump is operated continuously at constant power.
Numerous other features, objects, and advantages of the invention will become apparent from the following detailed description when read in connection with the accompanying drawings.